To put it another way, the Poser background is just something to display on your screen in the places where there is nothing visible in your 3D scene.

Since the Poser background is not a 3D object in your scene, it's actually unrealistic to include it in a render. But, just like your screen, a jpg render export must show something there, since it can't be transparent; so it uses the Poser background, or black, or white, depending on what you choose in the render options.

If you want to render a 'real' background to your scene, then you should put a 3D object there, with a suitable texture applied. In the real world, there is always something to be seen in every direction, which is why I now nearly always use an environment sphere with either a procedural or mapped image texture. In any (outside) scenes with reflective or refractive materials this usually gives better realism, since there is always something there to be reflected/refracted, just like in the real world.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)